two main ones
1. Scale or other deposited on the inside of the tube resulting in localized hot spots which then fail from over heating.
You combat this with manic levels of water purity control. Many trace elements in the water are monitored down to ppb levels and the DO, pH, and conductivity are also controlled.
2. External high temp corrosion attack. Usually in the form of sulidation or carburization.
This is combated by both control of firing conditions and by using protective coatings on the boiler tubes. This is usually a weld overlay or laser sintered Ni alloy coating.

Read EPRI papers on the subject.

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Plymouth Tube

Now that someone else had answered, I'll throw in my original thought on the subject, the main cause of boiler tube leaks is the existence of boiler tubes. I believe you can reduce them to some level, but beyond that you will always have to live with a certain level of leakage.

If that question could be answered in 1 paragraph, it would put EPRI and 200 consultants out of business. A partial list is ( for a 500 MWe unit) :
-water chemistry issues, UDC under deposit corrosion, hydrogen damage
-operator error, maintaining fires when the unit has lo-lo water level
-incorrectly designed furnace waterwall , including circulation issues ( for drum type boilers)and thermal hydraulic sensitivity issues for supercritical units
-liquid phase coal ash corrosion of superheater and reheater tubes for high sulphur coal-fired units
-incorrect dissimilar metal weld design at the ferritic-to-austenitic transition in superheater and reheater tubes
-incorrect spray attemporator design or control method
-oxide scale buildup within ferritic reheater tubes onus units that do not have a steam to reheater bypass system
-creep and oxidation damage to ferritic tubes that are being overheated for various reasons
-graphitization of CS superheater tubes and headers upstream of the 1st spray attemporator
-cyclic operation of a boiler that was designed strictly for base load operation

"Whom the gods would destroy, they first make mad "